Control Pressure Switch - S1

The EBCM provides the control pressure switch circuit (CKT 848, EBCM pin 13) 5 volts at all time. When the brakes are not applied, S1 is closed completing a path to ground for CKT 848 and holding EBCM in 13 at 0 volts ground.

When the brakes are applied enough to generate approximately 400 psi in the booster circuit, S1 opens, removing the path to ground and forcing CKT 848 to 5 volts. The EBCM monitors voltage on pin 13 to determine the position of the control pressure switch.